Mobile phones, which have been widely applied both in people's daily life and work, relieve spatial and temporal constrains to communication and provide great convenience for people's life and work. However, the communication environment is complex and ever changing, among which chaotic environment noise is included. Environment noise will seriously degrade the speech quality of calls on the mobile phones. Therefore, the speech enhancement techniques have significant implications for noise suppression on the mobile phones.
At present, the usually used speech enhancement techniques include single-microphone spectral subtraction speech enhancement technique which is also referred to as single-channel spectral subtraction speech enhancement technique or may be referred to as single-microphone denoising technique. Such technique may suppress steady noise only, without significant suppression effect for non-steady noise, for example, voices of the people around you in malls, supermarkets or restaurants.
Another preferred denoising technique is multi-microphone array speech enhancement technique which is also referred to as multi-microphone denoising technique. Such technique further includes the following two types.
A first type is where a fixed directionality is formed by means of multi-microphone signal processing. That is, sound from one specified direction is retained and sound from other directions is suppressed. When the directionality formed by such processing points to the mouth of the user, the purpose of protecting the speech and suppressing the noise around is realized.
A second type is where the speech is distinguished from noise by the energy difference of signals received by a plurality of microphones for the purpose of denoising. FIG. 1 is a diagram showing locations in which a plurality of microphones are mounted, in the solution of distinguishing the speech from noise by the energy difference of signals received by the plurality of microphones for the purpose of denoising. As shown in FIG. 1, there are two microphones, i.e., a main microphone 1 and an auxiliary microphone 2. The main microphone 1 is mounted on the bottom of a mobile phone close to the human mouth, and the energy of the speech signals received by the main microphone 1 from the user is quite high. The auxiliary microphone 2 is mounted on the top of the mobile phone far away from the human mouth, and the energy of the speech signals received by the auxiliary microphone 2 from the user is low. As shown in FIG. 1, the energy of the external environment noise received by the two microphones is very close to each other. Therefore, by comparing the difference of energy of signals received by the two microphones, it may be judged whether a signal is a speech signal or noise signal. As a result, the speech component is retained while the noise component is suppressed, and the signal to noise ratio is thus increased.
Compared with the single-microphone denoising technique, the multi-microphone denoising technique has the advantage that it may effectively suppress the non-steady noise. However; the multi-microphone denoising technique has strict constrains to the way of using a mobile phone by the user. That is, the way of holding the mobile phone by the user will be constrained. For the first type of technique utilizing the directionality, the user must keep holding the mobile phone in a way of pointing to the human mouth, otherwise the human mouth will deviate from the direction specified to be protected if the user holds the mobile phone in a different way. In this case, the speech will be suppressed as noise from other directions. The same is for the second type of technique utilizing the difference of energy, where the user is also required to hold the mobile phone with the main microphone kept close to the human mouth while the auxiliary microphone kept away from the mouth, as shown in FIG. 2 that is a diagram illustrating a state when the user holds a mobile phone normally. When the user holds the mobile phone abnormally, for example, the mobile phone is held horizontally as shown in FIG. 3 that is a diagram illustrating a state when the user holds a mobile phone abnormally, as the distance from the main microphone to the mouth is close to the distance from the auxiliary microphone to the mouth, the energy of the speech received by the two microphones is close to each other. In this case, the speech will also be suppressed as noise.